The present invention relates generally to digital data storage, and relates more particularly magnetic shift register memory.
Magnetic shift register memory that utilizes the manipulation of magnetic domains to store digital data has been proposed to potentially replace magnetic disc storage and even solid state non-volatile memory (e.g. flash memory). By stringing tens, or even hundreds, of magnetic domains (each domain representing one bit of digital data) along a shift register column (i.e., a magnetic column), the magnetic shift register memory efficiently uses the third dimension on the semiconductor chip to increase storage density and reduce cost per bit.
To perform read/write operations, a selected magnetic domain (as determined by address inputs of the memory) must be moved adjacent to a read/write device. Additional space (referred to as “reservoirs”) must be allocated above and below the data section of the magnetic column in order to prevent data loss due to overflow as the entire train of domains moves up and down the magnetic column.
Conventional magnetic shift register memory requires electrical current in the range of one to ten miliamperes to move a magnetic domain. Since the resistivities of common magnetic alloys are in the range of ten to fifty micro-ohm-centimeters, a considerable voltage drop results across the magnetic column during moving operations. This limits the number of domains that can be moved in a single move operation (i.e., by a single application of electrical current).
Moreover, conventional magnetic shift register memory requires at least three semiconductor switches per magnetic column: a first switch to selectively read one bit, a second switch to selectively write one bit, and a third switch to move the magnetic domains along the magnetic column so that any particular bit is accessible for read/write operations. Since writing and moving both involve electrical currents on the order of miliamperes, at least two of these three switches will occupy substantial area in the magnetic shift register memory. Thus, a significant amount of space that could otherwise be dedicated to data storage (i.e., to magnetic domains) is occupied by the semiconductor components, and the storage density of the magnetic shift register memory cannot reach its full potential.
Thus, there is a need in the art for a method and apparatus for a magnetic shift register memory device with improved storage density.